In vertical continuous annealing furnaces a single strand of cold rolled steel strip passes through several zones for heating, soaking and cooling, to recrystallization anneal and perform associated quenching and overageing treatments. For sheet steel annealing with overageing, the annealing cycle typically lasts 5-10 minutes. Strip speed in these furnaces can be as high as 450 mpm for sheet gauges and 650 mpm for tinplate gauges, as dictated by productivity considerations. The length of the furnace is minimized by passing the strip up and down (sinusoidally) over driven support rolls.
The strip moves through the furnace under tension to ensure good conformance to the driven support rolls, and, in combination with roll contours and steering mechanisms, to prevent excessive lateral strip motion leading to mistracking. The application of tension to the strip at high temperature also pulls out cold rolling shape defects through plastic elongation, the extent of which depends on the tension applied, on the steel's deformation resistance, and on the time during which the tension acts on the steel while it is soft enough to be deformed by normal values of strip tension.
Conventionally, strip tension inside continuous annealing furnaces is most simply controlled by pulling the strip between entry and exit bridles to generate the uniform tension profile. Strip tension can be controlled locally along the furnace by regulating the speeds of individual rolls relative to the strip speed, to step tension up or step tension down to appropriate levels. This procedure will be illustrated below.
Strip tension may also be regulated in discrete zones by using bridles inside the furnace. A bridle is a combination of two or more juxtaposed rolls positioned so as to maximize surface contact between the strip and at least one of the rolls, the latter being a driven roll. In these conventional schemes, tension is regulated at predetermined levels as measured by load cells, which provide a measure of the vertical or horizontal force (i.e., total load) on various support rolls. The appropriate total load used in a particular furnace section depends on strip cross-section (width and thickness), strength (depending on temperature, state of recrystallization and chemical composition), and the need for elongation flattening. The load is limited by the need to prevent creasing, over-necking (the width reduction associated with elongation) and strip breaks. The soaking section is the most critical area for tension control, because the yield strength of the strip is lowest there, typically about 1,000 psi for ultra-low carbon steel at 850.degree.-900.degree. C., making it most susceptible to tension effects.
The range of total load required in a furnace which processes a wide range of strip cross-sections and grades (composition and annealing temperature) makes precise control at the low end of the range difficult because the "dead band" of the best load cells, typically .+-.1 percent of full rated load, represents a large fraction of the total load needed for small cross-sections and soft grades. Harmonic strip flutter also causes actual strip tension fluctuations which broaden the band of uncertainty in load cell measurements. The accuracy of load cell regulation is further limited by the difficulty in distinguishing small changes in strip load in a total load cell signal imposed by strip load and roll weight.
1.1 Analysis
The tension pattern through a vertical annealer, and particularly for one with galvanizing capability, is one with high tension at the entry and exit ends and low tension in the middle section where the strip is hot and plastic.
Strip enters the furnace, from the cold mills where it is reduced up to 85% with very large induced stresses which are not uniform, resulting in irregular flatness across the strip width, and with various frequency of such defect lengthwise of the strip. Since such strip enters the furnace cold, its contact with the conveyor rolls is irregular, and high tension is required to increase its contact area to avoid slippage and sideways mistracking. This condition is highly aggravated by the thermal difference between the conveyor rolls which are near furnace temperature and the cold strip. Because of thermal conductivity those portions of the strip with short fiber length in good contact with the roll overheat compared to those portions of long fiber length. While this condition tends to ultimately correct strip shape when the strip begins to yield, it further affects tracking and the possibility of strip collapse, or heat buckling, later in the furnace.
The cold strip over the hot rolls further cools the portion of the roll in contact with the strip by conduction and radiation. The portion of the roll not in contact with the strip remains near furnace temperature and hence its diameter growth by thermal expansion is greater. To avoid gross mistracking of the strip due to subsequent concaving of the roll, the roll ends are tapered in cold condition. This requirement presents two other problems; namely, a stress rising point where the taper initiates, and a greater temperature difference across the sheet. This latter condition is further aggravated on a strip width change of larger size whereby the width addition contacts a portion of the roll hotter than the original extended center portion.
As the strip travels in this entry section of the furnace its temperature increases and some flattening, or removal of stresses, occurs as its yield point lowers due to temperature. When the strip temperature reaches a point where extension begins to occur the strain rate (function of tension) must be significantly decreased to avoid over-extension and consequent narrowing of the strip which would occur at the strain rates required at the furnace entry described above.
In the heating zone, the conveyor rolls in prior practice have been powered only to overcome the roll inertia upon speed changes. This practice does not provide for lowering the required high entry tension to the required low tension at the soak zone. Thus, bridles are used at the entry of the soak zone which abruptly changes the tension, FIG. 5. This practice is unsatisfactory however, since during transient changes of speed which occur very often, product on the high tension side of the bridle reaches peak temperature promoting heat buckles or coil breaks before the heating controls can respond.
When the strip has reached it aim setpoint temperature, it is held at the temperature for a period of time to allow all the carbon content to recrystallize, and to bring all portions of the strip across its width to the same temperature as far as possible due to the discrepancies above.
During this time final flattening of the strip is obtained by extension of the strip. This extension, however, should be carefully controlled as tensions, or strain rates, which are too high can cause heat buckles, and can over-extend the sheet causing more narrowing than necessary to flatten. Excessive narrowing requires more width at the pickle line and is more difficult to keep in commercial tolerance.
On both sides of the holding section the strip is at a temperature where both elastic and plastic extension occur. If extension and narrowing are to be kept at a minimum and controlled more easily, these areas should be kept at a lower strain rate (tension) to minimize the plastic or permanent extension and to keep the permanent extension more controllable.
The rolls in these areas again should be designed as multi-rolled bridles or a series of bridles to accomplish the required tension changes in stepwise fashion. While designing in this fashion requires more horsepower and more individual control than is the custom, expense can be justified in the material cost savings of the controlled narrowing.
The exit end of the annealer, following cooling to a nonplastic temperature range, requires a high tension to provide a very stable passline for coating in the case of galvanizing, and to prevent strip flutter causing uneven cooling and scratching in the highly dynamic final cooling sections of both annealers and galvanizers.
As the very critical soaking zone is sensitive to all changes of tensions, particularly those induced during changes of line speed, this section should be considered as the master speed section of the processing line such that all transient errors in the drive system are driven to the exit and entry ends, thus minimizing the magnitude of such transients in the process section. To accomplish this as well as provide the tension buildup, all rolls in this section should be designed as a multirolled bridle.
1.2 Flatness Defects
Tension plays a small part in the generation of flatness defects as long as it is applied and changed correctly with operating practice. The type of steel, its temperature and time at temperature dictate the stress required for a given extension required for flattening a given incoming shape and I value. Roll crowns for tracking are dictated by furnace type and design and if properly designed especially at taper break points contribute minimally to defects. The primary cause of defects is non-uniformity of temperature.
Temperature differences across the width in the heating section are fairly negated by the high yield strength of the strip which allows large elastic changes. Some differences do exist due to the uneven contact of cold strip to hot rolls which can be alleviated somewhat by roll shields. These resultant differences are, however, mostly removed in the soaking section with sufficient time to recrystallize the carbon content.
Heat buckles are caused almost entirely by subjecting hot strip to cold rolls and this can be highly aggravated by nonuniform strip temperature. This phenomenon occurs mostly in the first cooling section. Heat buckles can occur in the soaking section if excessive tension is used in conjunction with other faults such as misaligned rolls, edge over-cooling by cold atmosphere distribution, or with full crowned or heavily tapered rolls.
Rolls in the cooling section are greatly influenced by the cooling medium temperature and by the walls which are also cooled by this medium. These cold rolls quench the strip where it is in heavy contact as opposed to much lesser cooling where there is light or no contact. The rolls are provided with surrounding electric heating elements to help overcome this cooling effect, and the rolls should be kept within 75.degree. F. of the strip temperature, if possible.
The rolls have a very high thermal inertia which cause shape problems on changes such as width or speed. Roll temperatures will stabilize in steady operation with the portion under the strip hotter than the other portions. If the succeeding strip width is larger, this larger portion will then contact a colder portion of the roll and over cool relative to other portions of this strip. This cooled portion is restrained from contracting by the remainder of the strip and becomes elongated, usually in the plastic state, and upon further cooling yields wavy edges. This condition may exist in about 4000 foot of strip before acceptable temperature difference of strip to roll is reached.
Whenever a gauge change occurs necessitating a line speed change, there is always a large temperature difference in the strip across the weld which may persist for 1200 feet on either side of the weld. Likewise, on line slowdowns, long portions of the strip will overheat due to the furnace inertia before coming back into control. When these temperature overshoots associated with speed change become too large, heat buckles will occur until the strip and roll temperatures converge to acceptable limits. The auxiliary roll heating elements are too slow reacting to alleviate this problem. Lowering the tension during these transitions will help, but may not cure the problem.
A similar problem can exist in the heating section on a line slowdown since the strip will reach temperature earlier in the furnace and hence in a position where the tension is higher than desired. If this tension (set for elastic flattening and now acting on plastic strip) is too high, excessive extension and heat buckling can occur.
Such changes as described can be anticipated and feed forward signals sent to the furnace sections controls to avoid or minimize the damage. Usually, however, this requires the use of a mathematical model as the changes are too numerous and fast for an operator to calculate and react.
The initial cooling of the strip on the rolls and by the cooling medium itself may cause the flatness defect called cross bow. When hot strip passes over a colder roll, the strip face in contact with the roll cools to a greater extent than the back face. If the temperature difference between strip and roll is too great, longitudinal camber will occur on the roll due to the contraction of the contact face. As the strip leaves the roll and is subject to tension stretching, the strip width will contract on the colder face more than that of the back face, and if the resulting strain is large enough to cause plastic deformation a cross bow will occur. Cross bow may also occur in like manner but reverse direction in the heating zones although these are usually in the elastic stage and are easily removed. However, it is possible, particularly above 500.degree. F., to occasion plastic deformation if the temperature difference between the strip and the roll is too great. Such bowing requires more extension in soak to remove.